Currently, a two-piece can used throughout the world is constituted of a can barrel which is formed by applying working such as DRD (Draw and Redraw) working or DI (Draw and wall Ironing) working to a steel sheet and a lid. With respect to a beverage can, there has generally been adopted a method in which, to satisfy a demand for the corrosion resistance, an inner surface of a can is covered with an organic paint after can-making so that the contents of the can and the inner surface of the can are protected.
On the other hand, recently, a laminate steel sheet which is manufactured in such a manner that a metal sheet is covered with an organic resin film by coating in advance before forming has been attracting attentions in view of preserving the global environment. In the laminate steel sheet, the film per se has lubricating property and hence, a lubricant which has been conventionally necessary at the time of deep drawing or ironing becomes unnecessary. As a result, the laminate steel sheet has advantages of the possibility of the lubricant washing step being be omitted and of waste water from washing not being produced. Further, a step of coating an inner surface of a can and a step of baking a coated film which have been necessary for protecting the contents and a surface of the steel sheet become unnecessary, thus giving rise to an advantage of carbon dioxide which is a greenhouse gas discharged at the time of performing a baking step not being generated.
In this manner, the can manufacturing method which uses the laminate steel sheet largely contributes to preserving the global environment and hence, the future increase in demand for laminate steel sheets is expected. In this method, however, there is a possibility that a new problem may arise that a Thickness of a coated film is locally decreased due to surface roughening of a steel sheet which constitutes a base after a can is manufactured so that the corrosion resistance of the steel sheet deteriorates due to breaking of the film, peeling off of the film or the like. Accordingly, a steel sheet which constitutes a base is required to have, as important characteristics, high formability which enables the steel sheet to withstand the high degree of working such as deep drawing or ironing, and a surface property that surface roughening does not occur on a surface of the steel sheet so that favorable adhesiveness with a film is ensured after a can is manufactured. With respect to surface roughening which is generated on a surface of a steel sheet which constitutes a base after a can is manufactured, there has been known that the finer the average grain size of the steel sheet before a can is manufactured, the more effectively the surface roughening can be suppressed. A large number of techniques have been proposed in the past with respect to a method of making a grain size finer. Further, as an application of such a method, there has been also disclosed a technique in which a size of grain is made fine only on a surface region of a steel sheet to which a working die is brought into contact, while a size of grains in a center portion of the steel sheet is made coarse and softened to decrease working energy.
Patent document 1 discloses a hot-rolled steel sheet which is used as a raw material for a cold-rolled steel sheet having favorable formability which has excellent die galling resistance at the time of deep drawing, a method manufacturing the hot-rolled steel sheet, and a method of manufacturing a cold-rolled steel sheet by using the hot-rolled steel sheet as a raw material. The steel sheet can enhance both the deep drawing property and die galling resistance simultaneously by using, as a raw material for the cold-rolled steel sheet, the hot-rolled steel sheet in which a rate of a grain size in the sheet thickness direction and a [111] crystal azimuth are properly adjusted. However, the hot rolling is performed at a temperature equal to or below an Ar3 transformation point so that a fact that a higher temperature control technique and the higher quality control are required compared to the prior art, a fact that a rolling load is increased due to the lowering of a finish roiling temperature and the like are named as drawbacks to be solved.
Patent document 2 discloses a steel sheet for DI cans which cause only a small number of cracks at the time of flange forming thus having excellent workability and having high can body strength after coating and baking, and a method of manufacturing the steel sheet for DI cans. The steel sheet for DI cans has the plural layered structuring table for DI workability, wherein in a sheet thickness surface layer portion, fine AlN is precipitated so that grains are made fine and grain boundary strength is increased thus enhancing workability in secondary working such as necked-in working and flange working, while a sheet thickness center layer is formed into a coarse-grain soft material through averaging treatment. However, since the can body strength after coating and baking is increased by leaving solid solution C in the steel sheet, the adjustment of a total C content in a steel making step, a winding temperature control in a hot rolling step with respect to such a total C content or the adjustment of a solid solution C content in averaging treatment in an annealing step becomes necessary thus becoming a factor which causes lowering of productivity.
Patent document 3 provides a cold-rolled steel sheet having excellent die galling resistance, excellent chemical convertibility and excellent spot weldability by performing continuous annealing in a carburizing atmosphere. Ultra low carbon steel is used as a base for maintaining favorable workability. A carbon concentrated layer is formed on a surface of the steel sheet by annealing the steel sheet in a carburizing atmosphere so that slidability is enhanced thus overcoming a defect of the ultra low carbon steel in which die galling liable to easily occur. However, continuous annealing in a carburizing atmosphere is indispensable and hence, it is necessary to introduce a new facility into a conventional facility.
Patent document 4 discloses a method of manufacturing a steel sheet for DI cans in which a Nb-doped ultra low carbon steel is used, a sheet thickness is set to 0.20 mm or less for making the DI can light-weighted, and an average grain size of an original sheet is set to 6 μm or less. By setting the average grain size of the original sheet to 6 μm or less while ensuring the favorable workability by using the ultra low carbon steel, surface roughening the original sheet after ironing a steel sheet to which an organic resin film is laminated can be suppressed thus ensuring the corrosion resistance of the steel sheet. However, ironing of the laminate steel sheet is performed without using a lubricant and a coolant and hence, hardening of the steel sheet accompanying the excessive refinement of grains causes an excessively large heat generation by working and this excessively large heat generation becomes a drawback from a viewpoint of the industrial production of steel sheets.